Traditionally, innate immune cells have been considered "non-specific" and thought to respond identically when re-encountering the same pathogen. In a mouse model of CMV infection, we have shown that NK cells have several features that are normally attributed exclusively to the adaptive immune system. After infection with MCMV NK cells expressing the activating Ly49H receptor, which directly recognizes the MCMV m157 glycoprotein, undergo extensive proliferation. After control of the virus, the population of Ly49H+ NK cells contracts, but generates long-lived "memory" NK cells capable of enhanced cytokine production and cytolytic activity upon secondary encounter with the pathogen. Additionally, we have shown that co-expression of inhibitory Ly49 receptors reactive with self-MHC class I ligands restrain the response of these Ly49H+ NK cells during viral infection, such that NK cells without a self-MHC class I inhibitory receptor dominate the immune response to MCMV. The evolutionary advantage of maintaining NK cells that lack self-MHC class I reactive inhibitory receptors might be to ensure optimal responses to viral infection, despite the potential risk of attacking healthy cells. The overall goal of this grant is to characterize the nature of "memory" mouse NK cells using the MCMV model, to determine whether mouse NK cells are capable of generating "memory" against other viruses, and finally, whether human NK cells specifically recognize human CMV and also possess traits similar to mouse "memory" NK cells. The specific aims are: 1) To characterize the molecular and functional characteristics of long- lived "memory" mouse NK cells responding to mouse cytomegalovirus and to establish whether vaccination of NK cells is possible. 2) To determine whether mouse NK cells can specifically respond to viruses other than cytomegalovirus and acquire "memory" traits. 3) To test the hypothesis that the CD94-NKG2C receptor on NK cells recognizes human cytomegalovirus, and that interaction of this receptor with its ligand leads to generation of "memory" NK cells. PUBLIC HEALTH RELEVANCE: Natural Killer (NK) cells are a type of white blood cell that kill tumors and virus-infected cells and protect the host from cancer and infectious diseases. Previously, NK cells were considered non-specific in their recognition of pathogens;however, in a mouse model we have shown that some NK cells can specifically recognize mouse cytomegalovirus and protect the host from lethal infection with this virus. In the proposed study, we will determine whether NK cells can be vaccinated to provide improved protection of the host against cytomegalovirus. We will also examine whether NK cells can also protect mice against influenza virus infection. In humans, 60% of the population is infected by human cytomegalovirus (HCMV), which in healthy individuals only causes mild, subclinical infection;however, is life-threatening in newborns and in immunocompromised individuals, including kidney transplant patients and AIDS patients. Therefore, we will also examine whether human NK cells specifically recognize HCMV and determine how NK cells recognize this pathogen.